import Swift
public func diff<T : Differentiable, R : Differentiable>(
  _ f: @escaping @autodiff (T) -> R
) -> (T) -> (R, (T.TangentVector) -> R.TangentVector) {
  return Builtin.autodiffGetJVP(f)
}

public func diff2<T : Differentiable, R : Differentiable>(
  _ f: @escaping @autodiff (T) -> R, _ x: T
) {
  diff(f)(x)
}

swiftc -parse-stdlib -emit-ir test.swift

Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file /Users/rxwei/Development/Swift/swift-tf-source/llvm/include/llvm/Support/Casting.h, line 248.
Stack dump:
0.  Program arguments: /Users/rxwei/Development/Swift/swift-tf-source/build/Ninja-ReleaseAssert+stdlib-Release/swift-macosx-x86_64/bin/swift -frontend -emit-ir -primary-file test.swift -disable-objc-attr-requires-foundation-module -target x86_64-apple-darwin18.2.0 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -parse-stdlib -color-diagnostics -module-name test -o -
1.  While emitting IR SIL function "@$s4test4diffyq__13TangentVectorQy_ACQzctxcq_xcs14DifferentiableRzsAFR_r0_lF".
 for 'diff(_:)' at test.swift:2:8
0  swift                    0x00000001064e0558 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x00000001064df528 llvm::sys::RunSignalHandlers() + 248
2  swift                    0x00000001064e0b72 SignalHandler(int) + 258
3  libsystem_platform.dylib 0x00007fff5b59cb3d _sigtramp + 29
4  swift                    0x000000010719f462 cmark_strbuf__initbuf + 117613
5  libsystem_c.dylib        0x00007fff5b45b1c9 abort + 127
6  libsystem_c.dylib        0x00007fff5b423868 basename_r + 0
7  swift                    0x0000000102d39d74 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) + 57876
8  swift                    0x0000000102d28192 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 7938
9  swift                    0x0000000102c33ae2 swift::irgen::IRGenerator::emitGlobalTopLevel(bool) + 674
10 swift                    0x0000000102d000db performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, swift::SourceFile*, llvm::GlobalVariable**) + 1067
11 swift                    0x0000000102d00616 swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, llvm::GlobalVariable**) + 86
12 swift                    0x0000000102b8b9bb performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 13739
13 swift                    0x0000000102b873a0 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2848
14 swift                    0x0000000102b391de main + 686
15 libdyld.dylib            0x00007fff5b3b308d start + 1
[1]    97501 abort      $SWIFT_NINJA_BUILD_PATH/bin/swiftc -emit-ir -parse-stdlib test.swift